The present invention generally relates to an apparatus for firing a discharge lamp and, in particular, relates to such an apparatus which reduces transients resulting from the firing of the lamp.
In many modern analytical instruments the optical segment of the instrument often includes an arc lamp which emits light within a selected spectrum of wavelengths. As one example, a conventional spectrophotometer utilizes a deuterium gas arc lamp for producing a continuous spectrum of wavelengths from about 190 nm to about 340 nm. Depending on the spectrum desired, lamps having other materials, such as Xenon, can also be used.
In effect, such lamps require two different voltages for proper operation. That is, a firing voltage and a sustaining voltage. Additionally, there may also be a heater for the cathode or electron emitter to which a low voltage is first applied for a short period of time to generate free electrons within the lamp. Thereafter, the firing voltage is applied across the lamp to fully ionize the gas therein. Once the gas is ionized, the firing voltage is removed and a sustaining voltage is applied to maintain the gas in its ionized state. Such a heater, if used, may need voltage continuously applied thereto unless the cathode remains hot enough due to ion bombardment.
A difficulty with such lamps is that the firing voltage and the sustaining voltage change, usually by increasing, over a period of time. The rate of change of these voltages with time is unpredictable. Further, these voltages vary with the frequency of use of the lamp, this variation is also unpredictable. Additionally, these voltages vary from lamp to lamp, although this variation is not as significant as the variations with respect to time or frequency of use.
Conventional lamp firing mechanisms overcome this difficulty by supplying the lamp with a firing voltage which is considerably higher than the highest firing voltage anticipated. For example, if a new lamp requires a firing voltage of about 300 volts and it is predicted that after a few hundred hours of operation the firing voltage will be about 400 volts, the firing voltage supplied, throughout the life of the lamp, will perhaps be set at about 800 volts.
At the present time, the full, and excessive, firing voltage is applied to the lamp instantaneously. For example, this voltage is often generated by charging a plurality of capacitors and then applied to the lamp by closing a switch. An alternative scheme which is also in present use is to provide a voltage potential, possibly via a plurality of capacitors, and then, via a switch, impress that voltage onto the primary side of a voltage step-up transformer, the secondary of which is connected across the lamp and thus instantaneously subjecting the lamp to the full and excessive firing voltage.
The instantaneous application of the excessively high firing voltage not only causes ionic erosion and deterioration of the emitting electrode surfaces in the lamp, thus reducing lamp life, but also causes a transient signal to be impressed upon the electronic circuitry of the instrument. Such a transient can often cause false counts and errors in the digital computing system as well as exceed the tolerable voltage limits of the remainder of the instrument circuits and cause considerable disruption and interference with the normal operation thereof. This transient problem is particularly severe in instruments containing sensitive semiconductor circuitry, such as microprocessors.